Road map data may be created using an aerial photograph or on-site survey result, which needs considerable costs and time. If a road management entity does not report a road change such as road opening or road closing, the road map data fails to continue to have a practical use.
Patent document 1 discloses a method for updating road map data using navigation devices mounted in vehicles. The navigation device detects a fact that a vehicle track departs from the road indicated by the road map data and sends the vehicle track to a road map data update system via a wide area communications network. The road map data update system then reflects the received vehicle track on the existing road map data.                Patent document 1: JP-2004-198997 A (USP-7024307)        
Road map data used in a navigation device typically defines a road as one or two string lines. Actual roads have a wide range of width from a very narrow road having a lane to allow only one vehicle to run to a very wide road having several lanes in one traffic direction. Therefore, it is very difficult to accurately determine whether a vehicle track departs from an on-map road indicated by road map data. Positioning system using GPS or dead-reckoning navigation has a detection error, which causes the vehicle track to depart from the on-map road. Thus, the method, which uses the vehicle track and the road map data used in the navigation device, might be inaccurate in updating road map data.